The present invention relates to a UHT system for heat treating temperature-sensitive food products, desserts or dessert-like products in particular, with a pre-heating zone which has at least one first heat exchanger, which is/are impinged by the food product to be heated via a product line on the one hand, and by a regeneratively generated first heating medium via a first circuit line on the other hand, with a high-heating zone joining the first heat exchanger upstream with respect to the flow direction of the food product and comprising at least one second heat exchanger which is/are impinged by the food product via the product line on the one hand, and by a second heating medium via a second circuit line on the other hand, with a first line section which forms a section of the first circuit line and which supplies the first heating medium to the first heat exchanger of the pre-heating zone, and with a second line section which forms a further section of the continuing first circuit line and which conducts the first heating medium away from the first heat exchanger of the pre-heating zone, wherein the product-conducting heat exchangers of the pre-heating and the high-heating zones are each one designed as tube bundle heat exchangers with each having at least one tube bundle, and wherein the respective tube bundle consists of a group of parallel connected inner tubes, each flown through by product at the inner side. Moreover, the present invention relates to a method for heat treating temperature-sensitive food products of the kind mentioned above in a UHT system.
As a UHT-method (UHT: Ultra High Temperature) with indirect product heating through heat exchange by means of a heat carrier medium on a wall, one understands a thermal product treatment, which is also called aseptic heating, in which nearly all micro-organisms, but at least all the micro-organisms leading to deterioration, that might grow up during the storage phase of the product at room temperature, are killed. Hence, all micro-organisms except some heat-resistant spores perhaps surviving the heating process must be killed. However, these must grow up only to a defined value at normal room temperature during the storage phase.
The indirect product heating through heat exchange on a wall may take place with so-called plate heat exchanger systems or also, like in the invention described below, with so-called tube bundle heat exchangers, in which the heat energy is transferred through the tube walls of a group of inner tubes. In this, the food product to be treated flows in the inner tubes, whereas a heat carrier medium, called also a heating medium in the context of the present invention, normally water or steam, flows in a reverse flow in the ring gap chamber of the shell which surrounds the parallel connected inner tubes. Such a tube bundle heat exchanger is known from DE 94 03 913 U1.
A known, commercially available UHT heating device with indirect product heating for making UHT milk includes a pre-heater in a so-called pre-heating zone for heating the standardized milk. In most cases, the milk is subsequently conducted over a so-called homogenizer for fine distribution of fat, and further pre-heated thereafter. A so-called pre-hot-holding follows for stabilizing the milk proteins. After a further heat exchanger, which is provided for the subsequent milk heating process, the proper UHT heating takes place thereafter, in a so-called heating zone with hot-holding, followed by cooling in a so-called cooling zone under heat exchange with a so-called “regenerative” heat carrier medium. In case that, by way of example, UHT cream is also to be made with the UHT heating device, a post-cooling zone is disposed after the cooling zone, which is not incorporated into the regenerative heat exchange.
In the following, as a “regenerative” heat carrier medium by which a so-called “regenerative” heat exchange is performed, such a heat carrier medium will be understood which is conducted in a circuit and which, with respect to the flow direction of the food product to be treated, takes up heat energy from the product behind a heater hot-holding device and transfers it to the product “regeneratively” before the heater hot-holding device. In the context of the classification “regenerative”, the quotation marks will be constantly omitted in the following. In principle, water serves normally as heat carrier medium, which is conducted in a circuit and which heats the milk in reverse flow at higher temperature, corresponding to the course of temperature-time in the milk's forward flow, and cools the same in the back flow of the milk, also in reverse flow. In this heat exchange, up to 90% of the applied energy can be recycled. In this, the UHT heater is exempt from this regenerative heat exchange, and the necessary residual heating takes place here by indirect heating, the water circuit being redirected (Principle of FINNAH, Ahaus; H. G KESSLER, Molkereiverfahrenstechnik, Munich-Weihenstephan, 3. Edition, 1988).
In DE 10 2005 007 557 A1, a method and a UHT system is described for producing an extendedly durable drinking milk, a so-called ESL-milk, wherein this UHT system has the features of a commercially available UHT system for making UHT milk which were shortly delineated above. It shall only be noted here that a regenerative heat exchanger of the heating zone follows after the regenerative heat exchangers of the pre-heating zone and a pre-hot-holding device at first, before the milk is treated in the non-regenerative heat exchangers of the UHT heating zone. As the ESL milk is produced at an altogether lower course of the temperature level than the UHT milk, measures are proposed for selective reduction of the effective surface area of the UHT heater and for reducing the cooling efficiency in the cooling zone by conducting the milk in bypass in these areas, in order to control the heating input at the side of the heat carrier mediums, and thus for the temperature adjustment at the product side between pre-heating and high-heating and in the cooling zone.
Particularly temperature-sensitive food products, like desserts or dessert-like products, with high viscosity in particular and, as the case may be, with ingredients containing solid matter like whole pieces, pulp or fibres, require accurate and prompt temperature adjustment of the food product leaving the pre-heating zone to the temperature conditions at the entrance of the high-heating zone, which cannot be realised with the UHT systems for UHT milk or the modified UHT system for ESL milk according to DE 10 2005 007 557 A1 that were shortly delineated above.
Moreover, in achieving the aims mentioned above, thermally and mechanically gentle treatment of the food product is necessary at the same time. It follows from the requirement of thermally gentle treatment that all the partial amounts of a food product to be heat treated in the manner discussed here must pass the required course of the temperature level synchronously and over the same time duration. In other words, this means that all the partial amounts are subject to equal thermal and fluidic conditions at equal dwell time.
Mechanically gentle treatment means that the mechanical loading of the food product is kept as low as possible. Such a loading occurs always then in particular when the food product is subject to shear forces. The latter occur notoriously at deviations, discontinuous cross section transitions, branchings and unions of flow ducts, which are frequently present in a UHT system of the kind described above.
DE 103 11 529 B3 is devoted to the branching problem of the flow in the inlet region of the tube support plates of a tube bundle heat exchanger (for instance DE 94 03 913 U1), as the same is preferably used in UHT systems. The conducive measures proposed under the task specified here relate to the branching of a food product into inner tubes of the tube bundle heat exchanger receiving a number of partial amounts of this product, wherein a displacer is provided amongst others, which is disposed axis-symmetric and concentric to the tube support plate. In this, the inner tubes are distributed over the entire circular area of the tube support plate, normally over more than a partial circle except a closely limited central region. Given these conditions, differently long flow paths for entering the inner tubes or leaving them, respectively, are thus present from the start in the inlet- as well as in the outlet region of the respective tube support plate, hence in the branching and in the union of the flow. Different dwell times for the partial amounts of the food product flowing through the respective inner tubes result already only through this. The narrow central region of the tube support plate remains free of an inner tube, because here, the displacer is either fixedly connected to the tube support plate or is supported in it as a freely movable displacer. The allocation on the tube area is subject to a limitation insofar, because the centre of the tube support plate must remain free of one or even several inner tubes. In case that a number of inner tubes is provided, which are arranged on one single partial circle, like this is shown for instance in FIG. 9, different dwell times of the food product result at least for the inlet- and outlet region of the inner tubes, and the approach of a heat carrier medium to the inner tubes at the outer side thereof is distributed significantly unevenly over the opening cross section impinged by the medium.
From DE 10 2005 059 463 A1, it is known to arrange a displacer in the inlet-and outlet region of a tube support plate of a tube bundle heat exchanger, as the same is already known from DE 103 11 529 A1. This displacer has a shaft portion extending in the direction of its symmetry axis, which is fixedly connected at its end turned away from the displacer to a connection arc or connection fitting joining the exchanger flange or the attachment stub. By this, the centre of the tube support plate is also available for arranging inner tubes there, through which however, the flow paths differ still more in the course of the branching and union of the flow than upon allocation of the tube support plate with free centre, and thus, even greater dwell time differences are created generally and systematically.
GB 2 131 673 A discloses a method and a device for UHT treatment of liquids, like milk e.g., which are said to provoke only minimal thermally caused damage of the milk. The UHT system has amongst others a heat exchanger in a pre-heating zone, which is impinged by the liquid to be heated via a product line on the one hand, and on the other hand by a regeneratively generated first heating medium via a first circuit line. With respect to the flow direction of the liquid to be heated, a high-heating zone joins downstream of the pre-heating zone and has a heat exchanger which is impinged by the liquid to be heated via the product line on the one hand, and on the other hand by a second heating medium via a second circuit line. The heat exchanger of the pre-heating and the high-heating zone is shown in each case only schematically; there are no statements regarding the nature of its construction.
From the document U.S. Pat. No. 4,997,662 A, a method and a device for the heat treatment of milk in the context of pasteurization are known, which takes place on a relatively low temperature level in a known manner. The milk that is stored in a feeding tank after machine milking is supplied to a heat exchanger for the purpose of its pasteurization, which is subdivided into three sections connected in series, and about whose remaining construction no further statements are made. The first section serving as pre-heater for the entering milk is regeneratively heated in reverse flow by the milk heated to the desired end temperature and recycled from a hot-holding device. The second section of the heat exchanger joining the first section is impinged by the pre-heated milk on the one hand, and on the other hand it is connected to a first heating medium for further heating, warm or cold water according to necessity. Finally, the heating of the milk to the desired end temperature of the pasteurization takes place in the third section, to which the milk heated in the second section is supplied on the one hand, and which is impinged by a second heating medium on the other hand, which is preferably heated electrically and conducted in circuit.
Furthermore, document EP 0 081 256 A1 discloses a method and a device for controlled heat treatment of a food product, for instance for sterilizing milk, in a regeneratively working heat exchanger, wherein the last heat exchanger of the pre-heating zone is designed in the form of an arrangement of three concentric tubes. The central annular chamber, limited by the central tube radially at the outer side, conducts the product flow, whereas the outer annular chamber, limited by the outer tube at the outer side, or the inner tube can conduct the regeneratively generated heating medium at option.
The aim of the present invention is to achieve accurate and fast temperature adjustment of the food product leaving the pre-heating zone to the temperature conditions at the inlet of the high-heating in a UHT system of the generic type, and at the same time, with an equal dwell time for all partial amounts of the food product, to ensure that the food product is treated in a particularly thermally gentle manner, and to keep the mechanical loading of the food product as low as possible.